1. Field of the Invention
This invention relates to a mechanical device for interlocking multiple electric power switches in a distribution system which by providing a mechanical output only upon the occurrence of mechanical movement of both of two mechanical inputs.
2. Background Information
There are applications where it is necessary to coordinate the operation of electric power switches. For instance, in electrical distribution systems it is often required to mechanically interlock two or more circuit breakers so that only certain closure combinations are possible. A common example is the use of two or more power sources which may have slightly different voltage, frequency or phase angles and whose simultaneous connection to the same distribution bus could produce a xe2x80x9cfaultxe2x80x9d current. To facilitate the use of mechanical interlocking schemes, each circuit breaker is equipped with an output indication device which produces a motion when the breaker is closed and an input device which trips the breaker open, or holds it trip-free, when it receives an input signal in the form of a mechanical motion.
Where the operation of two switches is interlocked, such as in the case of a transfer switch for connecting alternate power sources to a distribution system, the output indication device on each switch is connected to the input or auxiliary trip device on the other so that only one switch can be on at a time. One arrangement for accomplishing this is disclosed in co-pending, commonly owned, application serial number 09/559,089, filed on Apr. 27, 2000. This system uses OR gates formed by doubled-ended levers, which when pulled at either end rotate the output shaft, although in the two switch combination only one input is utilized.
One of the most complex mechanical interlocking schemes involves three interlocked circuit breakers, any two of which can be closed at once. An example of its use is a xe2x80x9cdouble-endedxe2x80x9d switchboard with two independent sources and a split main bus than can be connected with a xe2x80x9ctiexe2x80x9d breaker. It is desirable to prevent simultaneous connection of both sources to the main bus, unless the tie breaker is open. But, if either of the main breakers is open, the tie breaker can be closed, thus feeding the split bus from a single source. This form of three-way mechanical interlock requires an AND logic element. Each breaker receives an input signal (motion) from the other two. Either signal alone will not operate the tripping device; it takes the combination of both inputs to produce the output (trip) response.
There is a need therefore for a mechanical AND gate to provide this logical response. This mechanical AND gate should have characteristics which make it simple and inexpensive to produce and install. Because the AND gate will be used less frequently, it is desirable that it be adapted to be interchanged with the simple OR gate currently used in simpler interlock arrangements. For proper operation, the AND gate should respond with no rotation if one input alone is present and with full rotation if both inputs are present. For design commonality it is also desirable that the input motions used with the AND gate be of the same magnitude and direction as those used for the OR gate with which it can be interchanged.
These needs and others are satisfied by the invention which is directed to a mechanical AND gate having a xe2x80x9cfloating pivotxe2x80x9d. More particularly, the mechanical AND gate comprises a pivot plate having an elongated slot. A first input coupling is mounted to the pivot plate at a point laterally offset to a first side of the elongated slot. A second input coupling is mounted to the pivot plate at a point laterally offset to a second side of the elongated slot. The output shaft of the gate extends transversely toward the pivot plate in alignment with the elongated slot. An output coupling mounted on but radially offset from the output shaft engages and is slideable relative to the pivot plate in the elongated slot. A first elongated actuator engages the first input coupling and is axially moveable between ON and OFF positions. Similarly, a second elongated actuator engages the second input coupling and is also axially movable between ON and OFF positions. The first and second input couplings are structured to only transfer force from the respective elongated actuators to the pivot plate with movement toward the ON position so that the pivot plate slides relative to the output coupling engaging the elongated slot, when only one of the elongated actuators moves to the ON position yet rotates to rotate the output coupling and therefore the output shaft only when both of the elongated actuators move toward their respective ON positions.
In the exemplary embodiment of the invention, the elongated actuators are positioned to move along substantially parallel strokes in opposite directions from their respective OFF to ON positions and the pivot plate has an OFF position in which the elongated slot is substantially parallel to the strokes of the elongated actuator.
More particularly, each of the input couplings is structured and positioned to provide lost motion between the associated elongated actuators and the pivot plate when the other of the elongated actuators moves to its ON position and translates the pivot plate, the lost motion being taken up as the other elongated actuator reaches its ON position. Preferably, the input couplings are slip couplings comprising a coupling element which slides relative to the elongated actuator to provide the lost motion and which engages an abutment surface on the elongated actuator to couple the elongated actuator to the pivot plate when the lost motion is taken up. This coupling element can be a swivel, including a swivel ring, through which the elongated actuator slides and seats against an abutment formed by a lateral shoulder on the elongated actuator.